1. Field of the Invention
The present invention relates to interfacing to segmentation-and-reassembly (SAR) devices in asynchronous transfer mode (ATM) switching apparatus.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows parts of conventional switching apparatus for use in an ATM communications network. The FIG. 1 apparatus comprises a plurality N-1 of physical-layer devices 2.sub.1, 2.sub.2, . . . 2.sub.N-1 and a corresponding plurality N-1 of traffic management devices 4.sub.1, 4.sub.2, . . . 4.sub.N-1. Each traffic management device 4.sub.i is connected to its corresponding physical-layer device 2.sub.i by a bi-directional data delivery path 6.sub.i.
The FIG. 1 apparatus also includes a switch fabric 8 which is, for example, a N.times.N cross-connect switching unit. The switch fabric 8 in fact has N input ports and N output ports. Each traffic management device 4.sub.i is connected by a bi-directional data delivery path 10.sub.i to a port-pair, made up of one input port and one output port. Accordingly, for the sake of simplicity, only the port-pairs are shown in FIG. 1.
The FIG. 1 apparatus also includes a segmentation-and-reassembly (SAR) device 12 which is connected to the port-pair N of the switch fabric 8 by a bi-directional data delivery path 10.sub.N. This SAR device 12 is in turn connected to an associated memory 14. Finally, the apparatus 1 includes a host processor (or switch controller) 16 which is connected to each of the traffic management devices 4.sub.1 to 4.sub.N-1 and to the memory 12.
In use of the FIG. 1 apparatus, the physical-layer devices 2.sub.1 to 2.sub.N-1 provide the apparatus 1 with a plurality of bi-directional ports (user-network interface or UNI ports) which are connected to physical-layer transmission lines. These physical-layer transmission lines may be, for example synchronous digital hierarchy (SDH) or synchronous optical network (SONET) transmission lines (ITU-T standard G.709), plesiochronous digital hierarchy (PDH) transmission lines (ITU-T G.703 standard), or fibre-distributed data interface (FDDI) transmission lines (4b/5b standard specified by the ATM Forum). In an ATM network, these transmission lines carry ATM cells in the form of a bit stream the format of which is dependent upon the particular physical medium used to provide the transmission line concerned. In the data-receiving direction (the direction in which cells are admitted into the switching apparatus) the physical-layer devices 2.sub.1 to 2.sub.N-1 convert the bit streams received at the UNI ports of the apparatus into streams of ATM cells which are delivered to the traffic management devices 4.sub.1 to 4.sub.N-1 via the respective data-delivery paths 6.sub.1 to 6.sub.N-1.
The traffic management devices 4.sub.1 to 4.sub.N-1 control the delivery of ATM cells to the switch fabric 8. The switch fabric 8 can provide up to N simultaneous data transfer paths, each path serving to permit transfer of data from a selected one of its input ports to a selected one of its output ports. The traffic management devices use these data transfer paths to exchange (switch) ATM cells synchronously. Overall control of the exchange process is normally performed by the host processor 16 which monitors the traffic flow conditions and selects the data transfer paths in successive time slots in order to provide a fair allocation of switch resources amongst the different cell flows passing through the apparatus.
After a traffic management device 4 receives an ATM cell through one of the data transfer paths provided by the switch fabric, it transfers that cell to its corresponding physical-layer device via the data delivery path 6. Each physical-layer device 2 converts the stream of ATM cells received thereby into bit streams suitable for transmission over the ATM transmission lines connected to the UNI ports of the physical-layer device concerned.
In an ATM network in which the FIG. 1 apparatus is used, most, but not all, of the ATM cell traffic carried is user data, whether that data represents voice signals, video signals, files, etc. However, some of the traffic carried by the network inevitably comprises control information such as signalling messages. Such signalling messages are required, for example, to establish a call. In addition, there may be a requirement for the host processors at different nodes of the ATM network (including the host processor 16 shown in FIG. 1) to communicate with one another using so-called "inter-host communication messages".
The signalling messages and inter-host communication messages are transferred across the ATM network in the form of ATM cells just like ordinary data traffic. However, the cells making up such messages are distinguished in some way from cells representing data, normally by the virtual path identifier (VPI) and virtual channel identifier (VCI) information contained in the header of each cell. The signalling messages and inter-host communication messages are generally too long to fit in the payload of a single ATM cell. Accordingly, at the source of each such message, the message is converted into a plurality of ATM cells which are then introduced successively into the network. This process is referred to as segmentation. At the destination of the message, and possibly also at any intermediate node of the ATM network at which it is desired to have access to the message concerned, the ATM cells making up the message are combined, in a process referred to as reassembly, to reproduce the original message. In the switching apparatus shown in FIG. 1, these segmentation and reassembly processes are carried out by the segmentation-and-reassembly (SAR) device 12 which is conventionally provided with its own dedicated port-pair (port N in FIG. 1) of the switch fabric 8. Thus, when the host processor 16 is informed that an ATM cell, whose VPI/VCI fields indicate that it belongs to a signalling message or an inter-host communications message, has been received by one of the traffic management devices 4 (the "source" traffic management device), the host processor 16 causes a data transfer path to be established from the input port of the switch fabric 8 to which the source traffic management device is connected (for example, input port 1 in the case of the traffic management device 4.sub.1) to the output port N of the switch fabric 8 so that the cell concerned can be delivered from the source traffic management device to the SAR device 12. The SAR device 12 then combines that cell with other cells belonging to the same message, using the memory 14, and, once the reassembly process for that message is complete, the message can be read by the host processor 16.
If the host processor 16 is the source of a signalling message or inter-host communication message it delivers that message to the memory 14 and the SAR device 12 then segments the message to produce a plurality of ATM cells. These cells are then transferred successively to one of the traffic management devices (the "destination" traffic management device), which is the traffic management device whose corresponding physical-layer device is connected to the transmission line through which the cells must be routed to reach the destination of the message. Under the control of the host processor 16, a data transfer path is provided for each successive cell of the message from the input port N of the switch fabric 8 to the output port of the switch fabric to which the destination traffic management device is connected. After reaching that destination traffic management device 4 the cells are then passed via the data delivery path 6 to the corresponding physical-layer device 2 and are output to the required transmission line through one of the UNI ports for onward transmission to the destination of the message.
In the FIG. 1 apparatus, the SAR device 12 is provided with its own dedicated port-pair (port N) on the switch fabric and the signalling messages and inter-host communication messages accordingly all pass through the switch fabric 8. Although the number of ATM cells involved in such messages is relatively small, as compared to the total number of ATM cells passing through the switch fabric, the need to pass the ATM cells making up the signalling and inter-host communication messages through the switch fabric inevitably leads to congestion in the switch fabric and reduces the number of opportunities to switch ATM cells representing user data. Furthermore, because one of the port-pairs of the switch fabric must be dedicated to the SAR device 12, the number of port-pairs available for connection to the traffic management devices is reduced by one. This ultimately limits the number of UNI ports of the switching apparatus as a whole.